Sorry to hear about your trouble. Hope you’re well on your way to being 100%.
Swimming used to help me tremendously when I had time. Currently my spare time is very, very limited. Is mountain biking an optimal choice for me?[/quote]
I can’t imagine mountain biking to take less time than a swimming session, so that could be tricky. Also, if you have any lingering shoulder, upper back, hip, knee, or ankle issues, supporting yourself on the bike over uneven terrain could be problematic.
Sparring, as in light contact with another person, is something I’d only do in an extremely controlled environment (like within the context of a class) and even then, it’s generally safer (not “safe”) with someone who you know and who knows you. A saying back when I taught and trained in a dojo was “white belts are more dangerous than black belts”, because they have less control, get over-excited/angry/ego-driven too easily, and tend to go harder than they should.
Not sure of your exact physical, financial, or time circumstances, but a well-designed strength training routine is one of the most reliable and effective ways to improve your physical condition. I believe you should make it a priority to do some kind of resistance training.
Bodyweight work or bands are going to be the least expensive route. Even a cheap gym membership like Planet Fitness could be worthwhile.